How could sequence.nextval be null in Oracle?

Question

I have an Oracle sequence defined like so:

CREATE SEQUENCE  "DALLAS"."X_SEQ"  
    MINVALUE 0 
    MAXVALUE 999999999999999999999999999 
    INCREMENT BY 1 START WITH 0 NOCACHE  NOORDER  NOCYCLE ;

It is used in a stored procedure to insert a record:

PROCEDURE Insert_Record
                (p_name    IN  VARCHAR2,                
                 p_userid  IN  INTEGER,
                 cur_out   OUT TYPES_PKG.RefCursor)
    IS
        v_id NUMBER := 0;
    BEGIN
        -- Get id value from sequence
        SELECT x_seq.nextval
          INTO v_id
          FROM dual;

        -- Line below is X_PKG line 40
        INSERT INTO X
            (the_id,            
             name,                        
             update_userid)
          VALUES
            (v_id,
             p_name,                        
             p_userid);

        -- Return new id
        OPEN cur_out FOR
            SELECT v_id the_id
              FROM dual;
    END;

Occasionally, this procedure returns an error when executed from application code.

ORA-01400: cannot insert NULL into ("DALLAS"."X"."THE_ID") 
ORA-06512: at "DALLAS.X_PKG", line 40 
ORA-06512: at line 1

Details that may or may not be relevant:

• Oracle Database 11g Enterprise Edition Release 11.2.0.1.0 - 64bit Production
• The procedure is executed via Microsoft.Practices.EnterpriseLibrary - Data.Oracle.OracleDatabase.ExecuteReader(DbCommand command)
• The application does not wrap the call in an explicit transaction.
• The insert fails intermittently - less than 1%

Under what circumstances could x_seq.nextval be null?

Answer 1

I'm pretty certain this will end up being an artifact of your code, or the .net driver you are using. I've knocked up a quick demo for you using pure SQL - PL/SQL and never get a lost sequence value. Incidentally the ref cursor you are using is probably unnecessary and likely impacts performance and readability of the code - my demo includes an insert_record2 procedure that consistently performs over 10% faster -in about 26s on my laptop vs 36 for the ref cursor version. I at least also think is easier to understand. You could obviously run a modified version against your test database complete with audit trigger.

/* 
demo for dbse 
assumes a user with create table, create sequence, create procedure pivs and quota. 

*/

drop table dbse13142 purge;

create table dbse13142(
    the_id number not null
,   name   varchar2(20)
,   userid number)
;

drop sequence x_seq;
CREATE SEQUENCE  X_SEQ NOCACHE  NOORDER  NOCYCLE ;

create or replace PROCEDURE Insert_Record
                (p_name    IN  VARCHAR2,                
                 p_userid  IN  INTEGER,
                 cur_out   OUT sys_refcursor)
    IS
        v_id NUMBER := 0;
    BEGIN
        -- Get id value from sequence
        SELECT x_seq.nextval
          INTO v_id
          FROM dual;

        -- Line below is X_PKG line 40
        INSERT INTO dbse13142
            (the_id,            
             name,                        
             userid)
          VALUES
            (v_id,
             p_name,                        
             p_userid);

        -- Return new id
        OPEN cur_out FOR
            SELECT v_id the_id
              FROM dual;
    END;
/


create or replace PROCEDURE Insert_Record2
                (p_name    IN  VARCHAR2,                
                 p_userid  IN  INTEGER,
                 p_theid   OUT dbse13142.the_id%type)
    IS
    BEGIN
        -- Get id value from sequence
        SELECT x_seq.nextval
          INTO p_theid
          FROM dual;

        -- Line below is X_PKG line 40
        INSERT INTO dbse13142
            (the_id,            
             name,                        
             userid)
          VALUES
            (p_theid,
             p_name,                        
             p_userid);
    END;
/

set timing on

declare
   c sys_refcursor;
begin   
for i in 1..100000 loop
   insert_record('User '||i,i,c);
   close c;
end loop;
commit;
end;
/

select count(*) from dbse13142;
truncate table dbse13142;

declare
  x number;
begin   
for i in 1..100000 loop
   insert_record2('User '||i,i,x);
end loop;
commit;
end;
/

select count(*) from dbse13142;
truncate table dbse13142;

Answer 2

Try doing a test case. Make a dummy table and insert 100,000 records using your sequence from the database. I'm betting you will have no problems. Next try inserting the same thing from your application.

Could this be caused by other issues such as an Oracle client mismatch?

Another solution that would fix the issue but not problem is to add a trigger on the table.
Before Insert on table on Dallas.X IF :the_id is null THEN SELECT x_seq.nextval INTO :the_id FROM dual; END IF;

Answer 3

I don't have priveldges to make comments yet, so writing this as an answer: Since you are using Oracle version >= 11.1, which allows sequences in PL/SQL expressions instead of in SQL, try this:

   v_id := x_seq.nextval;

Instead of this:

 -- Get id value from sequence
    SELECT x_seq.nextval
      INTO v_id
      FROM dual;

Or, although I've heard doubts / pitfalls when using ".currval", maybe omit the separate assignment of v_id and only use this code?:

 -- Line below is X_PKG line 40
        INSERT INTO X
            (the_id,            
             name,                        
             update_userid)
          VALUES
            (x_seq.nextval,
             p_name,                        
             p_userid);

        -- Return new id
        OPEN cur_out FOR
            SELECT x_seq.currval the_id
              FROM dual;

Sorry I don't have an 11g instance handy now to try this out.